Init from the current stacked branches

[adjerbetian]

If I already have some stacked branches

  c0      main
 | c1
 | c2     feature/a
 | c3     feature/b
 | c4     
 | c5     *feature/c

gh stack init should take all those branches into account (create a stack of feature/a > feature/b > feature/c.

Today, it only proposes to use the current branch (feature/c) as the first layer.

The only way to create the full stack is to use git stack init feature/a feature/b feature/c manually.